diff --git a/CHANGES.txt b/CHANGES.txt
index c33506e799..f202b5880c 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -147,6 +147,8 @@ RELEASE  VERSION/DATE TO BE FILLED IN LATER
       path munging if a non-default %TEMP% was defined (as in moving to
       a Dev Drive). Also some cleanup.
     - Improve the wording of Configure methods.
+    - Add unit test cases for AppendUnique, PrependUnique - verify behavior
+      if existing value already contained more than one of added value.
 
 
 RELEASE 4.9.1 -  Thu, 27 Mar 2025 11:40:20 -0700
diff --git a/SCons/EnvironmentTests.py b/SCons/EnvironmentTests.py
index 9f93fd2e91..6330665f59 100644
--- a/SCons/EnvironmentTests.py
+++ b/SCons/EnvironmentTests.py
@@ -1769,7 +1769,11 @@ def test_AppendENVPath(self) -> None:
         env1.AppendENVPath(
             'MYPATH', r'C:\mydir\num\one', 'MYENV', sep=';', delete_existing=True
         )
-        # this should do nothing since delete_existing is false (the default)
+        assert (
+            env1['MYENV']['MYPATH'] == r'C:\mydir\num\two;C:\mydir\num\three;C:\mydir\num\one'
+        ), env1['MYENV']['MYPATH']
+
+        # this should do nothing since delete_existing is false
         env1.AppendENVPath('MYPATH', r'C:\mydir\num\three', 'MYENV', sep=';')
         assert (
             env1['MYENV']['MYPATH'] == r'C:\mydir\num\two;C:\mydir\num\three;C:\mydir\num\one'
@@ -1786,8 +1790,8 @@ def test_AppendENVPath(self) -> None:
     def test_AppendUnique(self) -> None:
         """Test appending to unique values to construction variables
 
-        This strips values that are already present when lists are
-        involved."""
+        This strips values that are already present when lists are involved.
+        """
         env = self.TestEnvironment(AAA1 = 'a1',
                           AAA2 = 'a2',
                           AAA3 = 'a3',
@@ -1800,7 +1804,8 @@ def test_AppendUnique(self) -> None:
                           BBB5 = ['b5'],
                           CCC1 = '',
                           CCC2 = '',
-                          DDD1 = ['a', 'b', 'c'])
+                          DDD1 = ['a', 'b', 'c'],
+                          DDD2 = ['a', 'a', 'b'])
         env['LL1'] = [env.Literal('a literal'), env.Literal('b literal')]
         env['LL2'] = [env.Literal('c literal'), env.Literal('b literal')]
         env.AppendUnique(AAA1 = 'a1',
@@ -1816,6 +1821,7 @@ def test_AppendUnique(self) -> None:
                          CCC1 = 'c1',
                          CCC2 = ['c2'],
                          DDD1 = 'b',
+                         DDD2 = 'a',
                          LL1  = env.Literal('a literal'),
                          LL2  = env.Literal('a literal'))
 
@@ -1832,6 +1838,7 @@ def test_AppendUnique(self) -> None:
         assert env['CCC1'] == 'c1', env['CCC1']
         assert env['CCC2'] == ['c2'], env['CCC2']
         assert env['DDD1'] == ['a', 'b', 'c'], env['DDD1']
+        assert env['DDD2'] == ['a', 'a', 'b'], env['DDD2']  # keep existing dup
         assert env['LL1'] == [
             env.Literal('a literal'),
             env.Literal('b literal'),
@@ -1851,6 +1858,9 @@ def test_AppendUnique(self) -> None:
         env.AppendUnique(DDD1=['e', 'f', 'e'], delete_existing=True)
         assert env['DDD1'] == ['c', 'a', 'b', 'f', 'e'], env['DDD1']  # add last
 
+        env.AppendUnique(DDD2=['a'], delete_existing=True)
+        assert env['DDD2'] == ['b', 'a'], env['DDD2']  # all existing instances deleted
+
         # issue regression: substrings should not be deleted
         env.AppendUnique(BBB4='b4.newer', delete_existing=True)
         assert env['BBB4'] == ['b4', 'b4.new', 'b4.newer'], env['BBB4']
@@ -2569,7 +2579,8 @@ def test_PrependUnique(self) -> None:
                           BBB5 = ['b5'],
                           CCC1 = '',
                           CCC2 = '',
-                          DDD1 = ['a', 'b', 'c'])
+                          DDD1 = ['a', 'b', 'c'],
+                          DDD2 = ['b', 'a', 'a'])
         env.PrependUnique(AAA1 = 'a1',
                           AAA2 = ['a2'],
                           AAA3 = ['a3', 'b', 'c', 'b', 'a3'], # ignore dups
@@ -2582,7 +2593,8 @@ def test_PrependUnique(self) -> None:
                           BBB5 = ['b5.new'],
                           CCC1 = 'c1',
                           CCC2 = ['c2'],
-                          DDD1 = 'b')
+                          DDD1 = 'b',
+                          DDD2 = 'a')
         assert env['AAA1'] == 'a1a1', env['AAA1']
         assert env['AAA2'] == ['a2'], env['AAA2']
         assert env['AAA3'] == ['c', 'b', 'a3'], env['AAA3']
@@ -2596,6 +2608,7 @@ def test_PrependUnique(self) -> None:
         assert env['CCC1'] == 'c1', env['CCC1']
         assert env['CCC2'] == ['c2'], env['CCC2']
         assert env['DDD1'] == ['a', 'b', 'c'], env['DDD1']
+        assert env['DDD2'] == ['b', 'a', 'a'], env['DDD2']  # keep existing dup
 
         env.PrependUnique(DDD1='b', delete_existing=True)
         assert env['DDD1'] == ['b', 'a', 'c'], env['DDD1']  # b moves to front
@@ -2606,6 +2619,9 @@ def test_PrependUnique(self) -> None:
         env.PrependUnique(DDD1=['d', 'e', 'd'], delete_existing=True)
         assert env['DDD1'] == ['d', 'e', 'a', 'c', 'b'], env['DDD1']
 
+        env.PrependUnique(DDD2=['a'], delete_existing=True)
+        assert env['DDD2'] == ['a', 'b'], env['DDD2']  # all existing instances deleted
+
         # issue regression: substrings should not be deleted
         env.PrependUnique(BBB4='b4.newer', delete_existing=True)
         assert env['BBB4'] == ['b4.newer', 'b4.new', 'b4'], env['BBB4']